1. Field of the Invention
The present invention relates to the valve structure, rotor structure and rotor bearing pre-loading of a radial piston pump.
2. Description of the Related Art
In the typical radial piston pump, a central rotor is rotatably mounted by bearings in a housing having a plurality of circumferentially spaced piston bores radially extending from the rotor. The bearing pre-load for the rotor typically is provided by one or more shims of appropriate thickness between a cap for the housing and a race of one of the bearings. The rotor itself is provided with an eccentric cam surface at a location adjacent the radial bores. Normally, no structure is provided to compensate for the dynamic imbalance caused by the eccentricity of the cam surface.
A plurality of pistons typically is provided, with one piston in each radial bore. The pistons are spring-biased radially inwardly towards the cam surface of the rotor. Roller bearings and a race typically are provided between the cam surface and the pistons. The housing also normally is provided with an inlet valve and an outlet valve on either side of each piston bore.
As the rotor rotates, the pistons follow the cam surface, which moves them radially in and out within the radial bores. As a piston moves radially inward, hydraulic fluid is drawn into the piston bore through the inlet valve, while the outlet valve is closed. As the piston begins to move radially outward, the inlet valve closes, the outlet valve opens and the fluid is expelled.
This structure has several disadvantages. Pre-loading the rotor bearings using shims can be time-consuming and difficult. A shim must be put in place, the entire package assembled, and tested. If the shim size is incorrect, the pump must be disassembled, a new shim pack put in place, the mechanism reassembled, tested, etc.
With the typical valve structure, valves extend in through either side of the housing. This usually requires bores drilled in from either side of the housing, and, together with the bearing structure of the rotor, typically requires a two-piece housing. The entire structure requires extensive machining and assembly time, and must be made of a high grade wear-resistant material to withstand wear in the piston bores.
The typical rotor has no counterweights to compensate for the offset of the cam surface. The rotor therefore is subject to undesirable stresses when it rotates.